The present invention relates to a method for presenting an observer a holographically reconstructed three-dimensional scene (3D scene), and to a holographic display device in which this method is implemented.
The field of application of the invention includes holographic display devices in sectors where a very detailed and realistic representation of 3D scenes is essential. This can for example be the medical, geological or military industry, in order to plan operations, drill-holes and other activities precisely and thus to reduce costs considerably and/or to minimise risks. In addition, such display devices may be used in the private sector, e.g. for games.
Holographic display devices which implement this invention always also exhibit a visibility region which lies in the transformation plane of the encoded computer-generated hologram (CGH) within a periodicity interval of this transformation. The reconstruction of the 3D scene can be observed from an eye position in the visibility region, where the visibility region is larger than an eye pupil.
Wherever 3D scenes are mentioned in this document, this may relate to either stationary or moving 3D scenes, always including a single three-dimensional object.
Holography allows a 3D scene to be recorded and optically represented using wave-optical methods. The 3D scene is encoded in the form of a CGH on a light modulator, which serves as a carrier medium and which comprises discrete, spatially controllable light modulator means. Due to the illumination with light waves which are capable of generating interference, each point of the encoded 3D scene forms a point of origin of light waves which interfere with each other, and which, as a resultant light wave front, spatially reconstruct the 3D scene as if it was generated by light propagating from a real object in space. The holographic reconstruction of the 3D scene is achieved in a direct-view display with the help of an optical reconstruction system and by illuminating a carrier medium with sufficiently coherent light.
When encoding a hologram, the information can be encoded through complex-valued wave fronts of the 3D scene such that the observer sees different perspectives or views of this scene when he moves relative to the reconstruction of the 3D scene. Parts of the spatial scene may be visible in one view, but hidden and thus invisible in another view. This depends on the actual observer position from which the 3D scene is observed, and on whether parts of the 3D scene are situated in the background or in the foreground. These perceptions must be taken into consideration when computing the hologram of the 3D scene, in order to obtain a realistic reconstruction. Computation of those parts which are invisible can be omitted.
In natural scenes, individual parts of the 3D scene are perceived at a different brightness from different positions, due to the surface texture and the effects of the illumination of the 3D scene by a natural light source. For example, a reflection of incident sunlight is only visible in a mirror to an observer who looks into the mirror from a certain direction.
More generally, object surfaces reflect light in an angle-selective manner or radiate light in an angle-selective manner. Such characteristics of a realistic representation of a 3D scene should also be represented by the reconstruction of a hologram.
Various patent applications filed by the applicant describe a holographic image representation method in which a small visibility region is sufficient for viewing the reconstruction. The visibility region can therein be as large or only a little larger than the size of an eye pupil. The reconstruction of the 3D scene is generated for a right observer eye and for a left observer eye. If the observer moves, the reconstruction of the 3D scene is tracked to his new position with the help of position detection and tracking means. If necessary, the hologram must be computed and encoded anew for the new position, if the new view differs significantly from the previous one.
The smaller the visibility region, the less the perspective view changes within the visibility region, because there is only a small margin of movement. In a visibility region which is exactly as large as the eye pupil, there is an almost stationary view of the 3D scene. It can then already be determined in a generally known manner in the input data for hologram computation, for example as rendered in a 3D graphics programme, which object points are visible and which are not in the visibility region. If the hologram needs to be recomputed due to the tracking to the observer eyes, the changed perspective is taken into account in that these input data are changed.
However, the visibility region is preferably a little larger than the eye pupil, i.e. it has e.g. twice the size of the eye pupil, for example in order to compensate inaccuracies of the position detection and tracking means. The eye pupil can move to positions within such a visibility region which require a different view of the 3D scene.
The hologram computation—as described in previous patent applications filed by the applicant—is always performed with the aim to evenly distribute the light intensity of the individual object points in the visibility region, so that the 3D scene is visible in a uniform quality within the visibility region. However, if the visibility region is larger than the eye pupil, this does not fully comply with the requirements specified above with a view to a realistic representation of the 3D scene.
Although the change of the perspective as such, i.e. the relative displacement of object points within planes which are situated at different distances to the observer, is already considered in a thus computed hologram, errors in the reconstruction may occur in this case if the observer moves outside the centre of the visibility region. These errors are due to the fact that natural variations in brightness, at which individual parts of a 3D scene are visible from different perspectives, are not considered in the reconstruction. As a special case of this, the fact must be taken into account during hologram computation that object points may become visible or invisible in sections of the visibility region due to changes in the perspective.
Other known methods for computing the visibility or invisibility of parts of an object for example combine object points so as to form geometrically shaped areas. Depending on the actual requirements, algorithmic computation methods are carried out with these areas with object precision or image precision.
It is the object of the present invention to generate the reconstruction of a 3D scene in a holographic display device with a visibility region which is larger than the eye pupil such that a realistic visibility and/or invisibility of parts of the reconstructed 3D scene is realised for an observer eye at any position within the visibility region. The visibility of the object points of the 3D scene shall be taken into consideration during hologram computation.
The object is solved according to claim 1 by a method for reconstructing a 3D scene,                where the 3D scene is separated into individual object points of which a computer-generated hologram (CGH) of the 3D scene is encoded on a light modulator (SLM) of a holographic display device,        where at least one light source illuminates the light modulator with sufficiently coherent light and at least one transformation lens transforms the light,        where a processor controls control signals for encoding and reconstructing, and        where the reconstruction of the 3D scene is visible from an observer plane within a visibility region,characterised in that        the processor generates a spatial point matrix for defining the position of individual object points in horizontal, vertical and depth directions, and assigns the object points with predetermined intensity and phase values which approximate the 3D scene,        a complex-valued wave front is computed for each individual object point within the visibility region, and the intensity values of the object points are multiplied with a visibility function which is assigned to the individual object points and are then added so as to form an aggregated modified wave front of the object points,        the modified wave front is transformed into the plane of the light modulator in order to compute modified control values for the object points, and        control signals are generated in a control means, in order to encode the CGH in the light modulator with the modified control values and to transform it into the visibility region so as to reconstruct the 3D scene.        